The present invention relates to storage devices for computer systems. In particular, the present invention relates to optical and magneto-optical storage devices.
Optical data storage disc systems are a promising technology for storing large quantities of data. The data is accessed by focusing a laser beam onto a data surface of the disc and analyzing light reflected from or transmitted through the data surface. In general, in optical storage systems, data is in the form of marks carried on the surface of the disc which are detected using the reflected laser light. The surface of the disc is patterned to represent data that is typically stored in concentric tracks around the disc. The data is read from the disc by positioning the beam of light over a particular track on the disc and sensing the intensity and polarity of the reflected light from the disc.
There are a number of different optical disc technologies which are known in the industry. For example, compact discs are currently used to store digital data such as computer programs or digitized music. Typically, compact discs are permanently recorded during manufacture. Another type of optical system is a write-once read-many (WORM) system in which a user may permanently write information onto a blank disc. Other types of systems are erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems read data by measuring the rotation of the incident light polarization due to the storage medium.
The above systems require a beam of light to be focused onto a data surface of a disc and recovering the reflected light with a sensor. Storage density is determined not only by the size of the markings on the data surface, but also by the size of the beam focused on the surface (i.e. resolution).
In order to increase areal density in an optical drive, the spot size of the optical stylus (i.e., the focused beam) can be decreased by either decreasing the wavelength .lambda. of light or increasing the numerical aperture (NA) of the focusing elements. Other optical techniques which either directly or indirectly reduce the effective spot size of the optical stylus are generally referred to as "superresolution" techniques.
For example, it is well known that the resolving power of a microscope can be increased by placing an aperture with a pinhole (having a diameter smaller than the focused spot size) sufficiently close to the object being observed. It was previously thought to be impractical to place a pinhole aperture in sufficient proximity to an optical disc moving at several meters per second. The goal to achieve such an effect led to the development of another form of superresolution-magnetically induced superresolution (MSR), where a pinhole-like optical readout aperture is formed in the media itself. MSR media, however, is much more difficult to manufacture than conventional M-O media and is highly sensitive to temperature variations.
As another example, tapered optical fibers are commonly used to achieve superresolution in near field scanning optical microscopy. In this case the tip of the tapered fiber acts as a sub-wavelength aperture which generates evanescent fields that can couple to the object of interest. Tapered optical fibers, however, have extremely small power transfer efficiencies. A 50 nm diameter tip fiber has a power transmission of roughly 10.sup.-6, which is 4-5 orders of magnitude less than what would typically be acceptable in an optical recording drive (due to constraints on laser power and media sensitivity).
U.S. Pat. No. 5,161,134 to Lee, entitled METHOD FOR INCREASING LINEAR BIT DENSITY IN MAGNETO-OPTICAL STORAGE MEDIA, is herein incorporated by reference. The Lee patent discloses that sliders (flying heads) can be used to position a slit at close proximity to the media surface, thereby enabling superresolution. However, to achieve super resolution using a slit optimized for reading data, low transmission of power to the media typically results. This in turn requires very large amounts of laser power to write data on the media.